The invention relates to an optical adjusting process in proximity printing wherein a mask is spaced at a short distance from a substrate on which an image of the mask structure is to be formed.
In semiconductor technology, the structures in a mask are transferred onto a substrate which may, for example, be covered by a light- or radiation-sensitive photoelectric layer which is partially exposed via the mask image. Prior to the exposure step and the subsequent development process, the structures on the mask must be precisely adjusted to the substrate surface and to the structures on the surface of the substrate, respectively. The finer the structures to be produced are, the more precisely the adjustment has to be effected. As adjustment aids, both the mask structure and the substructure on the substrate are generally provided with adjustment marks which are made to coincide in the adjusting process.
Two processes are commonly employed for adjustment. The first is the so-called contact print process wherein the mask is directly placed onto the surface of the substrate. In this case, the structures of the mask and of the substrate surface are essentially located in one plane, so that an image formation of simultaneously high definition of adjustment marks in the mask and adjustment marks on the substrate surface in a common image plane via an optical system is unproblematic. In another known process, the so-called projection masking, the mask and the substrate are not made to register, but are located in different receiving means separated from one another. An image of the structures in the mask is then formed on the surface of the substrate by means of an elaborate optical system.
The so-called proximity printing is a variant of the contact print process. In this case, an extremely close distance between the surface of the substrate and the mask arranged above it is maintained so as to prevent manual damage to the mask and the structures in the substrate surface. With this process, it is possible to substantially increase the service life of the masks put into operation. The component yield may also be substantially improved on account of the avoidance of mechanical damage. The distance between the mask and the substrate surface is, for example, 20 to 100 .mu.m. The depth of definition of the objective employed for imaging of the mask structures and the structures of the substrate surface in a common image plane should then be chosen such that an image of the structures of both object planes used is formed with as high a definition as possible in the image plane. This requirement can, however, no longer be maintained if an image of very fine structures is to be formed, and a high magnification of the adjustment marks in the common image plane is necessary.